The present invention is directed to a fast mixing condensation nucleus counter for use in determining the physical characteristics of aerosol particles.
Atmospheric particles influence climate change, radiative transfer, visibility, and air quality. Atmospheric aerosols include particles that are emitted directly to the atmosphere and those that are formed in the atmosphere by the reactions of gaseous pollutants and certain natural compounds. At high concentrations, they become the haze that reduces visibility a becomes a health hazard. Aerosols also play an important role in the global atmosphere. They scatter sunlight back to space, producing a cooling effect that partially offsets the warming induced by greenhouse gases such as CO2.
Aerosol measurements characterize the size, concentration and composition of particles suspended in the atmosphere. The ability to measure the concentration and size distribution of fine particles is essential to the understanding of the dynamics of aerosols in the atmosphere, in combustion systems, or in technological applications. The importance of characterizing fast transient aerosols has increased in recent years. For example, rapid transients in aerosol systems can arise due to dynamic response, such as in diesel engine particle emissions, or as a result of high speed traversing through different air masses, commonly a problem in airborne measurements. A continuing focus of aerosol research, then, is the development of measurement methods that have the time and size resolution necessary to resolve rapid aerosol dynamics in the atmosphere and in technological systems.
Detection and analysis of aerosols using a condensation nucleus counter (CNC) is well known. The CNC is also used as the primary detector for obtaining particle size distributions, for example in scanning electrical mobility spectrometers (SEMS), also known as scanning mobility particle sizers (SPMS). However, traditional CNC designs have slow detector response times, limiting the speed at which particle size distributions can be obtained, and thus rendering them impractical for obtaining time sensitive particle size distributions.
The condensation nucleus counter detects particles by condensing a vapor on the particles to grow them to large enough size that they can be counted optically. This measurement involves four steps: i) the production of sufficient quantities of vapor; ii) creation of the supersaturation necessary to activate the particles; iii) maintenance of the particles in the supersaturated state long enough to grow to detectable size; and iv) detection of the grown particles. The time required for a CNC to respond to changes in the aerosol concentration is constrained by the sum of the relevant times.
Another problem with these traditional CNCs is that stable flow recirculations are created in these systems. Stable flow recirculations operate to randomly trap some of the sample particles within the CNC. Thus, while some particles immediately exit the mixing region and enter the detector, other particles continue to recirculate inside the CNC and randomly exit at some later time, introducing an exponentially decaying distribution of delays between the time a particle enters the CNC and when it is detected. This was not a problem for early uses of CNCs, but has important consequences when such detectors are used for time sensitive measurements. In particular, the distribution of delay times smears scanning DMA size distribution measurements so the full potential of SEMS systems has not yet been realized. These stable flow recirculations create mixing and detection delays of up to 1 s, making scans shorter than 3 s impractical in these CNC systems.
In these traditional CNC designs, the aerosol sample is first passed through a saturation chamber wherein a sufficient quantity of vapor-laden gas is produced, and then to a condensation chamber for supersaturation and growth. In later designs, the sample aerosol bypasses the saturation chamber and is fed directly into the condensor where it mixes, under laminar flow conditions, with a pre-saturated flow of gas from the saturation chamber. This simple plumbing change eliminates the time delay associated with vapor production step above, and increases the detection speed of the CNC dramatically. For example, in a CNC using the original design, such as the TSI Model 3010, a typical particle size distribution scan (with data inversion to correct for smearing of the data) can be taken in 30 to 45 s. Meanwhile, scans up to 10 times faster can be obtained with ultrafine CNC (UCNC) devices, such as the TSI Model 3025, utilizing the saturation chamber bypass design.
While scanning times are faster in these UCNC systems, such UCNC devices generally have a very small aerosol flow rates, up to 33 times smaller than the standard CNCs, reducing the count rate obtainable with these detectors and making such devices practical only for aerosols with extremely high number concentrations or long sample times. This is particularly true at the low end of the particle size distribution where the charging efficiency of the spectrometer is low. As a result of the low signal strength of such devices, particles in a single mobility channel must be scanned for a longer time, either by reducing the scan rate, or by summing the counts acquired during a number of scans. While either of these solutions will increase count rates, both of these solutions also increase the length of time needed to obtain a scan, rendering the device less than ideal for obtaining particle size distributions where small fast transients are involved.
An alternative design for continuous-flow CNCs is the mixing CNC (MCNC). In this instrument a cold aerosol flow is mixed with a comparable flow of hot, vapor-laden gas. The mixed gas then passes from the mixing chamber into a chamber that provides sufficient residence time for the supersaturated particles to grow to optically detectable sizes. In these MCNC systems, rapid, nearly adiabatic mixing is facilitated by making the mixing region turbulent. Turbulent mixing can achieve compositional homogeneity quickly and without the use of a cooler. However, until now, large mixing chamber volumes have been employed to prevent thermophoretic deposition of the aerosol particles in the mixing chamber. The large mixing chamber volumes employed in these MCNC systems also create stable recirculation zones within the mixing chamber, resulting in long residence times for the aerosol in the mixing chamber rendering these MCNCs too slow for use as a DMA detector.
Accordingly, a need exists for a system that provides a fast response CNC which would allow accelerated SEMS measurements by reducing the residence time of aerosol particles in the system while maintaining high sample flow rates that enable high count rates by the detector.
The present invention is directed to a fast mixing condensation nucleus counter (FMCNC), for use in obtaining particle size distributions of fast transient aerosol systems over a wide range of particle sizes. This invention utilizes the turbulent mixing technology of the MCNC systems to provide fast particle growth and high sample flow rate and signal strength, but restricts the size of the mixing chamber to minimize the detector delay associated with traditional MCNC systems. This invention is also directed to novel methods for obtaining particle size distributions of fast transient aerosol systems over a wide range of particle sizes using the FMCNC of the invention.
In one embodiment, the invention is directed to a fast mixing condensation nucleus counter comprising a detector and a mixing condensation device having a mixing chamber adapted to allow gas to flow within it along a preselected path to an outlet, wherein the outlet directs the gas flow to the detector. The mixing chamber has an inlet for introducing vapor-laden gas into the chamber and at least one nozzle for introducing a sample gas having particles entrained therein into the chamber. The inlet and nozzle are arranged such that the vapor-laden gas and sample gas mix turbulently. The mixing chamber is configured such that the particles are distributed throughout the mixing chamber and move through the mixing chamber at a substantially uniform relative velocity. In an alternative statement of the invention, the mixing chamber is configured such that adjacent particles within the mixed gas flow move through mixing condensation chamber substantially together.
The fast mixing condensation nucleus counter also comprises a growth tube positioned between the outlet of the mixing chamber and the detector, wherein the growth tube is adapted to allow the mixture of vapor-laden gas and sample gas to flow along a preselected path to the detector such that the mixture is given sufficient time to allow the vapor-laden gas to condense on the particles entrained in the sample gas and grow the particles large enough for optical detection by the detector.
In a preferred embodiment, the mixing chamber comprises two nozzles positioned opposite one another (antipodal) and transverse to the vapor-laden gas inlet. In this embodiment a gas distribution manifold evenly divides the sample gas flow between the two nozzles.
In another embodiment, the invention is directed to a fast mixing condensation nucleus counter where the counter comprises a mixing chamber as described above, a gas distribution manifold and a saturation chamber. The gas manifold is positioned between the source of sample gas and the saturation chamber, and the saturation chamber is positioned between the gas manifold and the mixing chamber. The gas manifold is adapted to evenly divide the sample gas flow into two gas flows and direct one flow into the saturation chamber and one flow directly into the nozzle of the mixing chamber. The gas flowing into the saturation chamber interacts with a vaporized working gas to produce a vapor-laden gas which is then directed into the inlet of the mixing chamber. In a preferred embodiment, the saturation chamber comprises a packed bed reservoir of vaporized working gas. In another preferred embodiment, the saturation chamber also comprises a temperature control apparatus, where the temperature control apparatus maintains the temperature of the vapor-laden gas at a first temperature and where the sample gas has a second temperature, and wherein the first temperature is greater than the second temperature such that when the vapor-laden gas and the sample gas mix in the mixing chamber, the hot vapor-laden gas condenses on the cold particles of the sample gas before reaching the detector. In another preferred embodiment the temperature of the vapor-laden gas is maintained at between 60 and 90xc2x0 C. and the temperature of the sample gas is maintained at room temperature.
In yet another preferred embodiment, the saturation chamber further comprises a filter positioned at the inlet to the saturation chamber such that the sample gas entering the saturation chamber is filtered to remove any particles entrained therein.
In yet another preferred embodiment, the fast mixing condensation nucleus counter further comprises a second gas manifold positioned between the filter and the saturation chamber. The gas manifold is adapted to divide the gas flowing through the filter into two gas flows, where one gas flow is directed to the saturation chamber and the other gas flow is directed through a bypass, and wherein the outlet of the saturation chamber and bypass are configured such that the flows mix prior to entering the inlet of the mixing chamber. In this embodiment of the invention, the division of the sample gas flow can be variably controlled such that by changing the ratio of gas flowing into the saturation chamber, the ratio of the vapor in the vapor-laden gas entering the mixing chamber can be adjusted.
In still another embodiment, the invention is directed to a fast mixing condensation nucleus counter wherein the sample gas source comprises calibration source comprising a nebulizer for producing a constant ultrafine sample gas flow rate and a tube furnace in fluid communication with the outlet of the nebulizer, wherein the tube furnace heats the sample gas to a constant furnace temperature. The tube furnace also having a furnace quencher wherein the quencher provides a source of filtered air such that the filtered air is injected into the outlet of the tube furnace at a specified injection rate to cool the heated gas. The sample gas source also comprising a differential mobility analyzer (DMA) classifier for sorting particles based on their size to produce a monodisperse ultrafine sample gas of specified particle size. In a preferred embodiment, the DMA classifier is either a cylindrical or radial DMA classifier. In another preferred embodiment, the sample gas source also comprises a charger disposed between the sample inlet and the inlet of the classifier. In yet another preferred embodiment, the charger employs a radioactive source, such as, a 210Po.
In still yet another embodiment, the invention is directed to a method for detecting the particle size distribution of particles entrained in a sample gas. The method comprises analyzing a sample gas using a fast mixing condensation nucleus counter as described above. In another embodiment, the invention is directed to a method for detecting the particle size distribution of particles entrained in a sample gas using the FMCNC described above in a fast scan mode.